Syndiotacticity and isotacticity involve two broad classes of stereospecific structure formations which may be involved in the formation of stereoregular polymers from various monomer units. Syndiotactic polymers, such as syndiotactic polypropylene, have a stereochemical structure in which the monomeric units have an enantiomorphic configuration in which the methyl groups on the asymmetrical carbon atoms follow each other alternatively and regularly in the main polymer chain. Isotactic polymers such as isotactic polypropylene generally are characterized as having the methyl groups on the repeating units with identical sequence configurations as contrasted with the alternating configurations of syndiotactic polymers. Such structures may be described by conventional and well-known graphical representations, such as Fischer projection and corresponding NMR pentad sequences as disclosed, for example, in U.S. Pat. Nos. 5,334,677 to Razavi et al and 4,522,982 to Ewen. While isotacticity and syndiotacticity are useful in defining these two broad types of crystalline polymer configurations, alternatives of both are known in the prior art. For example, so-called stereoblock polymers, such as disclosed in the aforementioned patent to Ewen, may be involved. Also a specialized form of isotactic polypropylene in which alternative polymer units achieve a random asymmetricity can be formed as stereoblock polymers which can be formed, for example, of alternating isotactic blocks. Various monomers which can be stereospecifically propagated include the ethylenically unsaturated monomers such as C3+ alpha olefins, such as propylene and 1-butene; dienes, such as 1,3-butadiene; substituted vinyl compounds, such as vinyl chloride or vinyl aromatic compounds, e.g. styrene; and vinyl ethers, such as alkyl vinyl ethers, e.g. isobutylvinyl ether or even arylvinyl ethers. As indicated above, the most significant application of stereospecific polymerization is in the production of isotactic or syndiotactic polypropylene.
Catalyst systems useful in the formation of isotactic polyolefins include the racemic bis-indenyl compounds of the type disclosed in U.S. Pat. No. 4,794,096 to Ewen. Those useful in the propagation of syndiotactic polypropylene and like syndiotactic polymers include stereorigid metallocenes and bridged cyclopentadienyl fluorenyl ligands, as disclosed, for example, in U.S. Pat. No. 5,334,677 to Razavi et al and U.S. Pat. No. 5,155,080 to Elder et al. A variation of such cyclopentadienyl fluorenyl ligand structures, which are substituted so as to produce a lack of bilateral symmetry, are disclosed in U.S. Pat. No. 5,036,034 to Ewen to produce hemi-isotactic polypropylene.
The catalysts most widely used in the formation of isotactic polyolefins take the form of bis(indenyl) compounds such as disclosed in the aforementioned U.S. Pat. No. 4,794,096. Other isospecific metallocenes are somewhat similar to syndiospecific metallocenes in that they are based upon cyclopentadienyl fluorenyl ligand configurations. One type of catalyst useful for the isospecific polymerization of olefins is disclosed in U.S. Pat. No. 5,416,228 to Ewen et al. Here, the ligand structure is configured so that one cyclopentadienyl group of a bridged ligand has a bulky group on one and only one of the distal positions of a cyclopentadienyl ring. Typical of such metallocenes is isopropylidene (3-tertiary butyl cyclopentadienyl fluorenyl) zirconium dichloride. These compounds, while similar to the syndiospecific metallocenes such as disclosed in U.S. Pat. No. 5,334,677 to Razavi et al, are, by virtue of the substituent group at the distal position on the cyclopentadienyl ring, characterized by a lack of bilateral symmetry. The metallocene catalysts may be supported on chemically inert solids including inorganic oxides such as silica.
Other isospecific metallocenes based on cyclopentadienyl fluorenyl ligand structures are disclosed in European Patent Publication No. 0881,236A1 to Razavi. Here, the ligand structures are characterized by bridged cyclopentadienyl and fluorenyl groups in which the cyclopentadienyl group is substituted at both proximal and distal positions. The distal substituent is preferably a bulky group such as a tertiary butyl group, and the proximal substituent is a less bulky group such as a methyl group which may be either vicinal or non-vicinal to the distal substituent. The fluorenyl group may be substituted or unsubstituted with up to eight substituent groups but preferably are unsubstituted at the positions which are distal to the bridgehead carbon atom. Specifically disclosed in EPO 881,236A1 are isopropylidene(3-tertiary butyl, 5-methyl cyclopentadienyl fluorenyl) zirconium dichloride and isopropylidene(3-tertiary butyl, 2-methyl cyclopentadienyl fluorenyl) zirconium dichloride. Similarly, as described above, with reference to the Razavi et al '677 patent, the metallocenes here may be supported on inorganic oxides with the preferred support being silica. In the Razavi EPO publication, the preferred support is silica having a surface area of between 200-700 m2/g. and a pore volume between 0.5 and 3.0 ml/g.
Yet another isospecific metallocene based upon bis(fluorenyl) ligand structures is disclosed in U.S. Pat. No. 5,945,365 to Reddy. Here, the ligand structure is characterized by two bridged fluorenyl groups with 1 or 2 substituents at distal positions on each fluorenyl group with one group of substituents being located transversely from the other with respect to a plane of bilateral symmetry extending through the bridge group.